The Gastrotheca guentheri is the strangest frog in the world. 200 million years ago it had a full mouth of teeth. The lower teeth evolved away, then suddenly reappeared millions of years later. It's the best evidence yet of re-evolution.

As far as frogs go, Gastrotheca guentheri is an oddball's oddball. The entire Gastrotheca genus comprises the so-called marsupial frogs, so named because they carry their eggs with them in pouches on their back. But the guentheri species is even more unusual, because it's the only known frog species with teeth on its lower as well as its upper jaw.

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What's even weirder is that this frog species originally had these teeth, then lost them, and then re-evolved them after 200 million years. It's some of the best evidence yet for the still fiercely debated question of whether species can re-evolve complex features that they once possessed but then lost. Dr. John Weins explains how he made this startling finding, and what it means:

"I combined data from fossils and DNA sequences with new statistical methods and showed that frogs lost their teeth on the lower jaw more than 230 million years ago, but that they re-appeared in G. guentheri within the past 20 million years. The loss of mandibular teeth in the ancestor of modern frogs and their re-appearance in G. guentheri provides very strong evidence for the controversial idea that complex anatomical traits that are evolutionarily lost can re-evolve, even after being absent for hundreds of millions of years."

Weins proposes this represent a "loophole" to the currently understood processes of evolution. The frog species lost its bottom teeth, yes, but the underlying mechanisms that would allow them to regrow the teeth remained in tact through those hundreds of millions of years. Weins explains why that makes a difference:

"What G. guentheri did was to put teeth back on the lower jaw, rather than having to re-evolve all the mechanisms for making teeth 'from scratch.' This 'loophole' may apply to many other cases when traits appear to re-evolve, such as in the re-evolution of lost fingers and toes in lizards."

"Re-evolution" still isn't a particularly well-established concept - hence the rather unimaginative name - but a lot of evidence is now piling up to support its existence. Beyond these strange frogs, scientists have discovered re-evolved wings on stick insects, renewed coiling in limpet shells, and, as we recently reported, lost and found digits in lizards and dinosaurs.